1. Field of the Invention
This invention relates to methods and apparatus for reducing pattern noise in the signals produced by a solid state image sensing array.
2. Discussion Related to the Problem
Solid state arrays of light sensitive elements have been proposed for use in various image sensing applications such as the primary image sensors in both television and still picture cameras and in various image analysis roles such as exposure control in photographic cameras and photographic printers. Generally, the signals produced by such arrays of solid state image sensors are subject to two types of disturbances generally referred to as pattern noise. The common feature possessed by these disturbances is that the disturbances are repeatable, and once measured can be removed from the signals. One type of pattern noise is characterized by different fixed amounts of bias in the signal produced by the sensor elements in an array. Dark current is a major contributor to this type of pattern noise which will be referred to hereinafter as "dark current pattern noise." The other general type of pattern noise is characterized by differences in sensitivity among the elements of the sensor array and will be referred to as "sensitivity pattern noise." Sensitivity pattern noise is caused mainly by manufacturing tolerances in the physical dimensions of the sensor elements and by variations in the transparency of electrodes and/or color filters disposed over the elements in the array. Apparent sensitivity variations may also result from non-uniform illumination of a scene that is viewed by the image sensing array. For example, if the scene is a projected image of a transparency, and the projection lamp has "hot spots" in its illumination pattern or the projection optics exhibit vignetting, the resulting effect on the signal produced by the image sensing array will appear as sensitivity pattern noise. Pattern noise is visible in the display of a signal produced by a solid state image sensing array as a pattern of randomly disposed light and/or dark areas superimposed on the image produced by the display.
In the prior art, two general approaches have been taken for correcting or reducing the effects of pattern noise. In the first approach, used most often, for example, in systems with linear image sensing arrays or in very expensive television camera systems, pattern noise is completely removed from the response of every element in the array. U.S. Pat. No. 3,949,162, issued Apr. 6, 1976 to R. M. Malueg, shows such a system for removing dark current pattern noise from the response of a solid state image sensing array. The image sensor is operated in the dark to generate the dark current noise pattern of the sensor. The output of the sensor, thus operated in the dark, is stored in a memory. Later, when the sensor is operated normally to sense an image, the values of dark current stored in the memory are subtracted from the photosignals produced by each element of the array. In this way dark current pattern noise is removed from the signal produced by every sensing element in the array. Similar approaches have been employed for removing sensitivity pattern noise. According to these approaches, the sensor is operated under controlled illumination conditions both in the dark and with non-imagewise illumination and the responses thereto recorded. As used herein, "non-imagewise illumination" means that there is no imagewise modulation of the light, however, there may be constant patterns of brightness superimposed on the illumination resulting in apparent sensitivity variations as described above. The dark responses are subtracted from the light responses to yield pure light responses exhibiting only the sensitivity variations. Percentagewise corrections required to produce a uniform response from all the sensor elements in the array are computed and the required corrections are stored in a memory. Later, during normal operation, dark current corrections are applied to the signals produced by the elements in the array, the dark current corrected signals are then multiplied by factors corresponding to the computed percentagewise corrections to equalize the responses of the elements in the array. For example, see U.S. Pat. No. 3,800,078, issued Mar. 26, 1974 to Cochran et al. and U.S. Pat. No. 4,032,975, issued June 28, 1977 to Malueg et al.
Excellent pattern noise correction is achieved and fine image detail is conserved according to the pattern noise correction methods outlined above. Unfortunately, the cost of the relatively large memories needed to store corrections for each sensor element can be very high, especially for two-dimensional image sensing arrays with large numbers of elements. The memory cost is prohibitive for certain applications such as amateur solid state video still cameras. Although providing excellent correction for the visible noise in the image, the total correction technique also corrects for noise that would have been neither visible nor objectionable in the image reproduced from the sensor signals.
In attempts to reduce the coat, size and complexity of noise correcting circuitry, an alternative approach taken in the prior art has been to detect and record the locations of those sensor elements whose signals are most severely afflicted with pattern noise, and during normal operation, to substitute, for the signals produced by the noisy sensor elements, a signal produced by a neighboring element or the average of a plurality of signals produced by neighboring elements. Both dark current and sensitivity variation pattern noise have been treated in this manner (see U.S. Pat. No. 4,167,755, issued Sept. 11, 1979 to Nagumo). This approach uses relatively simple circuitry and requires a relatively small memory for storing the locations of "bad" sensors. Visibility of the pattern noise in the picture is reduced, however, at the expense of fine image detail. Image detail is sacrificed because some information produced by the sensor, albeit in the presence of noise, is thrown away when a signal from a noisy picture element is replaced by the signal produced by a less noisy neighbor. It has been our experience with experimental image sensors that the instances where the photo information from a sensor element is totally lost or destroyed due to pattern noise are relatively rare exceptions, and in the vast majority of cases, the photosignal from a noisy sensor element is either fully recoverable or recoverable in an attenuated (clipped) form.
The problem we faced, therefore, was to provide a noise reducing circuit for a solid state image sensing array that embodied the advantages of both the prior art approaches, i.e. the ability to preserve image detail while requiring a relatively small memory size and relatively simple circuitry.